DESCRIPTION: Because post-meiotic spermatids are matured within a germline syncytium, an essential late step in spermatogenesis is the separation of spermatids into individual cells. Spermatid individualization requires an extensive reorganization of cyst cytoplasm and membranes. In Drosophila, this process is mediated by a specialized cytoskeletal membrane complex that is assembled at the rostral end of the spermatid bundle, around or within the bundle of 64 spermatid nuclei, then translocates along the length of the flagellar bundle, a distance of nearly 2 mm in D. melanogaster. As it moves caudally along the sperm bundle, this individualization complex mediates the reorganization or assembly de novo of plasma membranes around each spermatid, and simultaneously extrudes most of the cytoplasm from between sperm tails. A simple screening procedure to identify male-sterile mutants displaying altered individualization complex structure and dynamics has been developed. One mutant identified in this screen appears to undergo a defective individualization process, in which the cytoskeletal component of the individualization complex is observed moving down the cysts, but no cytoplasmic or membrane rearrangements indicative of successful individualization are observed. Preliminary analysis by electron microscopy indicates a specific defect in the formation or maturation of one of the two mitochondrial derivatives that extend along Drosophila sperm tails. Previous studies had implicated the same organelle in the formation and/or activity of the individualization complex. The phenotype of this mutation will be documented extensively by light and electron microscopy, with particular attention to the formation and structure of mitochondrial derivatives throughout the spermatogenic pathway. Additional alleles of the gene will be isolated, and molecular analysis of the locus will be initiated based on a transposon-tagging strategy using the existing P-element insertion allele. The product of the locus will be identified by germline transformation and complementation of mutant alleles. Because the development of spermatids within a syncytium is a conserved feature of spermatogenesis, the analysis of this gene and others with related phenotypes may provide a path to the identification of cellular components specifically required for sperm individualization in other organisms, including man.